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The unrest underlined escalating tension in the Islamic Republic six months after the election plunged the oil producer into turmoil and exposed splits within the clerical and political establishment.

"Protesters chanted anti-government slogans when carrying his body," Jaras said, adding fierce clashes also erupted in the central cities of Isfahan and Najafabad.

The same website earlier said security forces fired tear gas to disperse opposition supporters in Tehran.

It said protesters set one police motorbike on fire. A plume of black smoke could be seen above the centre of the city, Jaras added, as police blocked streets in the area and clashes intensified.

A witness told Reuters there was a heavy presence of both security forces and opposition backers in central Tehran, a city of around 12 million people.

"Police prevented groups of protesters from joining each other," she said.

Other witnesses said thousands of pro-government Iranians were also gathering in central Tehran.

A mobile phone network appeared to be down, Tehran residents said.

The authorities have warned the pro-reform opposition against using a two-day Shi'ite Muslim religious mourning ritual on Dec. 26-27 to revive protests against the clerical establishment.

The post-election unrest is the biggest in the Islamic Republic's 30-year history. Authorities deny opposition charges that voting was rigged to secure victory for President Mahmoud Ahmadinejad and have portrayed the protests as a foreign-backed bid to topple the Islamic establishment.

It has complicated a long-running international dispute over its nuclear programme, which the West believes may have military ends, not just civilian purposes. World powers have set an end-of-year deadline for Iran to agree a U.N.-drafted deal to ship most of its low-enriched uranium abroad in exchange for fuel for a Tehran research reactor.

The turmoil has also set back tentative efforts towards a rapprochement between Iran and its longtime foe the United States instigated by US President Barack Obama when he took power in January.